A technique to measure physical quantity using a fiber Bragg grating (FBG) has been known. For example, a system to search damage of composite material used for the material of aircraft using FBG and an arrayed waveguide grating (AWG) has been proposed (e.g. see Japanese Unexamined Patent Application Publication No. 2007-232371). A device to detect vibration or an elastic wave using FBG has also been proposed (see Japanese Unexamined Patent Application Publication No. 2010-112867, Japanese Unexamined Patent Application Publication No. 2008-046036 and Y. Okabe, J. Kuwahara, K. Natori, N. Takeda, T. Ogisu, S. Kojima, and S. Komatsuzaki, “Evaluation of Debonding Progress in Composite Bonded Structures Using Ultrasonic Waves Received in Fiber Bragg Grating Sensors”, Smart Mater. Struct., Vol. 16, No. 4, pp. 1370-1378, (2007)). Particularly in the field of development material for aircraft, applying FBG to measuring strain is anticipated.
In the case of a conventional micro-displacement measuring device using FBG, electric noise must be removed by equalizing and filtering a detection signal in order to secure the signal to noise ratio (SNR). However in order to equalize a detection signal, measurement must be performed for a number of times required for equalization. This makes the measuring time long. Further, if filtering is performed using a low pass filter (LPF) or a high pass filter (HPF) to remove noise, useful data may be removed as well.
Further, depending on the measurement principle of a micro-displacement measuring device, an expensive device, such as a Fabry-Perot filter, is required. In particular, a wavelength variable filter, such as the Fabry-Perot filter, requires a mechanism to mechanically drive an optical element. This leads to a further increase in manufacturing cost. Furthermore, the wavelength variable filter has an optical element driving unit, which makes it difficult to measure an object that is subject to vibration, including an aircraft, a windmill and a bridge.